This study is an observational cohort to identify the etiologies of acute febrile illness (less than or equal to 2 weeks), which are thought to be endemic or novel in a certain area. Therefore, there are no hypotheses that require sample size calculation to be tested. It is expected that this study will enroll a minimum of 200 patients per site: approximately 100 adults and 100 pediatric cases. The final sample size is therefore estimated to be 1600. However, because of the exploratory nature of the study, the investigators will conduct interim analyses. Based on such interim data, the investigators may determine that additional subjects, likely focused in certain disease groups, may need to be enrolled to better study questions of particular interest. Total enrollment will however be capped at 3200 volunteers if a decision is made to increase the sample size.